Humans are exposed to genotoxic agents through a variety of sources. Micronucleus (MN) formation is an endpoint which can be used to detect DNA damage resulting from clastogenic or aneugenic mechanisms. A sensitive and high throughput system to measure human blood for MN would have a myriad of biomonitoring applications. For example, such a system could provide information regarding the chromosome damaging activity of new drugs undergoing clinical trials, chemotherapy regimens, as well as accidental radiation or chemical exposures. A goal of the current application is to optimize methods developed during Phase 1 that allow for the flow cytometric measurement of MN in human reticulocytes (RETs0. Experiments are planned to test whether MN-RET measurements represent a sensitive indicator of cytogenetic damage. To achieve this goal, our Phase II aims are to. 1. COMPLETE optimization of cell staining and handling procedures. 2. DEVELOP biological standards to aid the calibration of flow cytometer parameters. 3. MEASURE the time-course and magnitude of MN-RET induction in radiotherapy and radio-plus chemo-therapy patients before and during the course of treatment. 4. MEASURE the time-course and magnitude of MN-RET induction in radiotherapy and radio-plus chemo-therapy patients before and during the course of treatment. 5. COLLABORATE with researchers in an environmental biomonitoring study of Chernobyl liquidators, clean-up workers and settlers. PROPOSED COMMERCIAL APPLICATIONS: The successful completion of this research project will enable Litron Laboratories to become an expert facility, capable of performing cytogenetic measurements on human blood samples on a fee-for-fee service basis. Additionally, by developing the necessary reagents into kit format, it will be possible to make this technology available to other laboratories having access to a single-laser flow cytometer. This technologically innovative technique has the potential to become an important clinical tool for measuring DNA damage carried by environmental exposures: radiation and/or chemotherapeutic therapies, aging and a myriad of other possible sources.